Thursday, February 06, 2014

Sands through the hour glass

A couple of charts for a bit of fun.

Recently we read about 102 year old Frenchman, Robert Marchand, who last week set a new best distance of 26.925 kilometres for the hour at the new velodrome in Saint-Quentin-en-Yvelines, France. It was widely reported in many journals and blogs. Here's a link to cyclingnews.com item but a quick Google search will show lots of reports. Chapeau to Robert!

Having been involved with a number of age group category hour records, I thought I'd chart all the current records, including the two outlier points of Marchand, and Chris Boardman.

Here they are in table form with details as at February 2014:



I included Chris Boardman's record in the men's list for reference, and because it was set using the then pursuit bike set up rules which are the rules that masters age category records are run under (although those rules have been modified somewhat since Boardman's phenomenal ride).

I note however that our famous centenarian used a regular (Merckx-style) bike set up, but let's not be overly concerned with that. Being upright, let alone riding is super stuff at that age!

Here are all the age group record holders plotted showing age and distance covered in the hour. Click on the pic to see a larger version.


For some added fun I drew a line linking Boardman's and Marchand's records. The slope is just a touch under 400 metres less per year of age. With the exception of the younger masters age categories up to about 40/45 years, the men's records seem to roughly follow that level of performance decline with age, perhaps with a decline closer to a metre per day of age.

There are fewer masters women's records and none past 66 years old, so it's harder to say if their rate of performance decline is comparable, so while eye-balling suggests the rate of decline is less severe than for the men, I'm not sure I'd draw too many conclusions from these data. There are so many variables, and as the ageless TV soap used to tell us... like sands through the hour glass, so are the days of our lives.

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Monday, November 04, 2013

Left Right Out of Balance

One of the more recent features to emerge with on bike power meters has been the reporting of something called "Power Balance". It's meant to provide an indicator of the split in power production between your left and right legs - an indicator of asymmetry in power output. It's a pretty simplistic indicator of what's going on with pedalling forces, and masks over much detail. It's also not all that clear whether it's of much value.

How's your power balance?
Power balance is now available via compatible ANT+ head units paired with power meters that supply power balance data (e.g. some Quarqs, Power2Max, Garmin Vector, Rotor). Other power meters can also provide such data via their own proprietary data recording systems (e.g. SRM, Polar/Look, MEP, Axis Cranks), and of course some indoor trainers have provided similar data for many years (e.g. Computrainer and Wattbike).

So if your Power Balance is reporting as 46%-54%, then it's reasonable to assume that means 46% of the total power output is coming from your left leg and 54% from your right leg. Except that maybe it's not.

So then, what is it really measuring?
Is it important/useful?
And is the L-R power balance data accurate?

What is Power Balance actually measuring?


You see, while some of the more popular ANT+ power meters are each reporting a power balance number, there isn't a standard as to what power balance actually means, and these meters are not reporting the same thing.

There are two main types of power balance data reported (there's actually more but I'm going to leave those others out for now), and which of these a power meter reports depends on where and how the forces are measured. I've decided to give each a name, it's possible others have already done the same and used a different name and quite possibly there are better ways to distinguish between each type. It's also possible I'm wrong with some details, and I'll be (happily) corrected if that's the case.

The issue comes about primarily because different power meters measure the forces at different points along the drivetrain, somewhere along the transmission from the pedals to the rear tyre. The dividing line is whether the measurement is done upstream or downstream of the spindle or bottom bracket connecting the left and ride side crank arms. Upstream means measurement of the forces applied to each crank arm, or to each pedal (or even cleats or shoes), and downstream means measurement of the forces applied to the crank spider, or chainrings, chain, rear cogs, rear hub, wheel or tyre.

You see, the downstream measurement locations cannot distinguish from which crank arm the force is being applied, whereas measurement done upstream on each crank arm or at each pedal can make that distinction (but they must measure both sides independently to do that). Downstream measurement of power balance is therefore split based on a crank's rotational location whereas upstream measurement of power balance is based on which side of the bike the forces have been applied.

1. Downstream power balance
This version of power balance is calculated from the power applied during the time when one or the other crankarm is forward of the bottom bracket, irrespective of which leg or crank arm is applying the forces. In other words, the left side power balance is the net contribution to power from both legs while the left hand side crank arm is forward of the bottom bracket, expressed as a percentage of the total power. Right side power balance of course must then be the same for when the right crank arm is forward of the bottom bracket.

This is the power balance values the Quarq and Power2Max reports, and also what the Computrainer, Wattbike and SRM systems report via their own data systems.

2. Upstream power balance
This version of power balance is calculated from the power applied to either crank arm for the entire pedal stroke. This version of left side power balance is then the net power applied by the left leg only to the left crank arm for the entire pedal stroke. And of course the same applies to the right side.

This is what Garmin Vector, MEP, Axis Cranks etc report.

1. and 2. are not therefore, measuring the same thing, and nor is one necessarily better than the other in its current simplistic guise.

As an excellent demonstration of the differences, Ray on his DC Rainmaker blog did a test showing the live power balance numbers reported from a Quarq (a downstream device) and the Garmin Vectors (an upstream device). Here's a link to the youtube video:

http://www.youtube.com/watch?v=k0i_jV9ygLI

It's quite obvious how much difference exists between these two versions of power balance. That also assumes of course that each was accurately reporting their version of left and right side data. I'll get to that later.

There is a little more to understand with these data, for instance because the cranks are a connected system, then what happens on one side is affected by what's going on with the other. So while we may see net torque and power reported from each each side (be it the upstream of downstream version of power balance), it is still masking what's actually going on. As yet, data streams with sufficient frequency are not available via ANT+ since it's constrained by transmission of data packets at 4Hz. To do that requires alternative means, which is what SRM's torque analysis, Wattbike and some other solutions provide.

Is power balance data important/useful?


In short, we really don't know. I think there will be times when such data may prove to be somewhat helpful, perhaps in assessing things like bike fits, but one needs to be careful with any assumption that achieving symmetry is the objective. It's not. Better performance is the objective.

And then we also need to learn how to interpret the difference between upstream and downstream power balance data.

So let me start by stating something already very well established in scientific study of pedalling.

Asymmetry in power production is normal and everyone will have a different L-R power balance. It's also well established that asymmetry is also variable and will vary with:
  • power output, absolute and/or relative
  • cadence (or torque)
  • fatigue
  • and likely a few other factors such as bicycle position, seated v standing and so on
Here are just a handful of links to study abstracts to emphasise this point about asymmetry being both normal and variable:
http://www.ncbi.nlm.nih.gov/pubmed/979569
http://www.ncbi.nlm.nih.gov/pubmed/10460126
http://www.ncbi.nlm.nih.gov/pubmed/17369798
http://www.ncbi.nlm.nih.gov/pubmed/21055708

There are others that go back to the 1970s but quick link abstracts are not available. This is not a scientific review, but you get the idea.

So, for instance, it's pretty common to see a different power balance at different power outputs as well as at the same power output but at different times during a ride.

So now that you have a power balance number and a trace of how that balance varies during your rides, what now? I mean we all have such asymmetries, some of us more than others, and yet it doesn't always appear to be a significant impediment to performance improvement, certainly not in my own case of a sizeable acquired pedalling asymmetry as shown in this item, but perhaps I'm the exception and not the rule.

Some asymmetries we might be able to address, and some we might not. Of those that we can, should we be concerned with them? Like I said, I'll leave that question open for others to address but it is my view that we should really only be concerned with those that will demonstrably lead to an improvement in performance, and I'd consider a reduction in potenital for injuries as an improvement (not that I am in any way implying there's an established causal link between power asymmetry and injury).

So far we really have no strong evidence either way to know whether this infomation provides us with any actionable intelligence. And so we progress instead with anecdote, personal experience and experimentation, belief, and all the biases and lack of controls that go with it. Anecdata if you will. Over time I'm sure better information will arise as more research is conducted into pedalling biomechanics.

For now, I'd put power balance data into the category of a curiosity, of limited practical value until some better research is conducted into its use and validity.

Accuracy


Another factor to consider is accuracy of the left-right power data. Don't just assume your shiny new Vectors are accurately reporting power balance or that your Computrainer spin scan data is correct either.

As yet, there really hasn't been much in the way of an assessment of the accuracy of left - right power data, nor for that matter any investigation of the accuracy of many power meter devices in the scientific literature for quite a long time. Please direct me to any if I've missed them.

Downstream power balance data
I would say that the downstream power balance data from a Quarq, Power2Max etc is quite likely to be about as accurate as the total power reported. This is because the same set of strain gauges are used to measure and parse the forces, and all that's required is a means to establish the crank's postion each revolution. Even so, this could use verification, and something as simple as checking the variance in left and right side static torque measurements can provide some decent clues.

Nevertheless, we also know that some ANT+ head units and meters often suffer this tendency to falsely repeat 2-3 seconds of power values when you stop pedalling, and these ghost power readings can readily result in quite sizeable discrepencies in overall power accuracy. Are such false power readings also affecting power balance calculations? If you tend to stop pedalling on one side more than the other, it may well be that such power ghost data is mostly attributed to that one side. I really don't know.

Upstream Power Balance data
The left-right upstream power balance data from a Garmin Vector could well be brilliant or it could conceivably be out by some margin. The other day I saw someone post on Facebook some ride data from their new Vectors. Nearly three hours of solid riding with a 61%-39% L-R power balance, and they were inclined to actually believe it. Knowing the rider, I know his asymmetry is not even close that level.

We already know from DC Rainmaker's blog review the accuracy of the Garmin Vector is affected by how tightly the pedals have been installed. This is summarised in this chart showing how the Vectors Ray was testing reported power relative to other power meters when they were installed into the cranks with different tightness:

Vector's accuracy as a function of how tightly the pedals are installed into the crank arms.
Chart from DC Rainmaker's blog

Given the total variance in reported power for both pedals can be of the order of around 10% depending on how tightly the pedals were installed, then I can readily imagine a large bias error in reported pedal balance would be very easy to create if each pedal is tightened differently.

Indeed it's quite possible a bias error already exists even if both pedals are installed to specification, simply because you have two separate measurement devices, each with its own error range.


So, even if you consider power balance data is of value, you'll need a means to verify its accuracy, as well as understand the difference between upstream and downstream power balance, lest you make poor decisions about a training or positional intervention.


Never fear, you could always wear a power balance bracelet and solve all your problems.


For those that are not aware, it's a forum custom that pink font indicates sarcasm.

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Monday, October 21, 2013

Crash test dummies

Following my Aero Testing for Dummies post the other day, I've been getting questions about aero testing services and when people can start getting tested, as well as general interest in progress with the project. I won't go into business plan detail but will post casual updates here every so often, and eventually when it's ready for full professional service delivery, I'll update our website (this is a primarily a personal blog, not a commercial one).

This is a sample of the comments:

Sounds like a terrific service. Good for you Alex, am sure it will be a big hit. Be cool to incorporate with a 'dynamic bike fit' service too - make a change to bar drop/saddle height etc, do a lap while you test comfort and aerodynamics. 
It's right up my alley - am trying to think of an excuse to get back to Sydney now.

This technology and service is a perfect complement for bike fit services.

I have no personal desire to do professional bike fitting, I'd rather provide aero test consulting services to professional fitters and/or work in tandem with them. Indeed I'd prefer a bike fitter be involved, so they can learn about the impact of their fitting and equipment decisions on the rider's aerodynamics, power and speed, and make the adjustments with the benefit of my analysis, feedback and advice. I'm sure over time they will get better for seeing such information, as will I.

I can't say much about what's in the pipeline but I expect the system may well ultimately integrate with professional fitting tools so that rider/bike fit and aero data can be managed for ongoing reference and development.

And similar to bike fitting, much of the quality comes with the knowledge and professionalism of those providing the service. The tools are an enabler and/or help with efficiency. The difference with this tool is the results are immediately apparent, objectively measured and precisely quantifiable.

Next up for me is what I've called Phase II - Demonstration testing

This is where I'll do some properly controlled testing over next month or so, but the emphasis will be on nailing the process, getting good aero results and giving people the opportunity to see it in action, and not so much on commercial considerations beyond refining operational processes for efficient use of testing time.

I already have some crash test dummies lined up.

Once I am satisfied with that I'll move to full professional service delivery, and provide some clearer guidance on services and pricing.

I'm hoping to get to Melbourne at some time before long and there's reasonable a chance I might be heading to Tour of Bright to work at the race and perhaps I could do something at the Wangaratta track around that time if anyone is about and logistics can be arranged - but that's just a thought bubble for now.

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Saturday, October 19, 2013

Aero Testing for Dummies

Just lately I've been having some fun with the Alphamantis aerodynamics testing system here in Australia.

In a nutshell, this system enables me to precisely assess changes in a rider's aerodynamics in real time while they are riding their bike.

The rider does not need to do anything special other than ride their bike around a suitable track. Any regular oval-like cycling track or velodrome will do, although an indoor velodrome is preferred as environmental conditions are far more predictable indoors and of course you won’t get rained out.

All that's needed is an ANT+ power meter and speed sensor on the bike, and a clear track to ride around, one where the rider can maintain position and does not need to brake.

Do a handful of laps, and we have your CdA baseline number. Then make a change to your bike position or change a piece of equipment (e.g. helmet or wheel), do a few more laps and we can tell immediately if you are getting an improvement, and by how much. Lather, rinse, repeat.

Test while riding, know if the position is rideable, how it feels at race pace, as well as whether or not it is faster for the power you have, and by how much.

Simple.

Here's a 7-minute long video showing a sample of it in action, with me explaining in voice over.



That's just a sample of the data capture, there are lots of other things but that's the essence of it.

I've been in a systems testing phase these past couple of weeks, and have successfully tested the system with two riders using different power meters (one an SRM, the other a Powertap) at the Dunc Gray Velodrome. Apart from Rod, who's data was featured in the video, the other rider was my mate Tony. It was only a systems test, not a properly controlled aero test, nevertheless we were able to quickly discern for Tony a difference between two good aero helmets.

The next phase will be to demonstrate for those interested in seeing it in action, in particular those with a professional interest in aerodynamic related cycling performance improvement such as bike fitters, coaches, squad development people, and organisations that have tracks. Australia has over 90 velodromes/tracks to choose from!

The system is portable, meaning I can set it up anywhere with a suitable track, so if there is sufficient interest and access to a clear track, I can travel. All I need is access to a regular 240V power outlet and maybe a table and chair for convenience.

How it works:

Power and speed information from your bike's ANT+ device is transmitted wirelessly to special software on my laptop which, along with various parameters (e.g. rider morphological data, track geometry data, rolling resistance and environmental data) and track timing tape data, calculates and plots charts of power output, speed, CdA, exact track position and so on. It does all this in real time.

There are lots of calculations going on underneath (viewable if desired) such as centre of mass speed, lean angles, precise lap distance actually ridden and so on. And more work is being done to further refine the already sophisticated physics modelling, e.g. modelling the intra lap variation in rolling resistance/tyre scrub, as well as integration with individual track timing data systems for even more frequent precise positional data.

The system also works with the Alphamantis "aerostick" device that can additionally capture the relative wind speed and yaw angles, enabling the software to parse out the effects of any wind during testing.

All the data is also captured for reporting, additional analysis, as well as replaying the data to the rider afterwards if desired.

It is, very, very cool.

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Monday, October 14, 2013

Power meter usage on the rise at Kona

The Kona World Ironman Championships is over for another year. As part of event registration, the organisers do a count of equipment choices, and this includes power meter brand if fitted to a competitor's bike.

So just for fun, I thought I'd look at the prevalence of power meter usage by Kona competitors for the last five years, including this year. I managed, with a little help from the wattage forum and Google, to locate the data and compiled them into table and chart form. Click on the images to see a larger version.



So while competitor numbers at Kona increased by 36% over the five years 2009-2013, the number of power meters installed on competitor's bikes over that same period increased by 174%!

We can see three major power meter brands: SRM, Powertap and Quarq, have dominated the Kona power meter landscape to date, with Quarq in particular showing strong growth over this period.

Power2Max have begun to make inroads in recent years, and we can also see the emergence in 2013 of newer power meter offerings from Rotor, Stages and Garmin. The old Polar power meter is a distant memory, and ergomo usage never really got going, although a few souls were still using them up to 2012.

It appears the overall solid increase in usage of power meters by those competing at Kona has largely been satisfied by uptake of newer power meter brands. It would be risky to assume this translates directly into general market trends for power meter usage but it certainly provides a good snapshot of the trends for Ironman athletes.

The data on total bike numbers and power meter counts were obtained from these website links by Lava Magazine (2010-2013) and Triathlete Magazine (2009):

2013 Kona IM Bike Count
2012 Kona IM Bike Count
2011 Kona IM Bike Count
2010 Kona IM Bike Count
2009 Kona IM Bike Count

Links were active and  available at time of writing this post.

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Monday, August 26, 2013

Resistance is futile, even for MTB

In February I posted an item, The sum of the parts, which discussed the energy demands of cycling, and explored the age old "wheel weight versus wheel aerodynamics" debate, and gave some examples of wheel choice by way of modelling the weight vs aero performance trade offs using a forward integration model to consider dynamic acceleration scenarios.

I'm going to return to dynamic modelling one day to explore more realistic acceleration power outputs than the constant power examples used, but not today. Modelling a dynamically changing scenario is a bit trickier than steady state (constant velocity) scenario, and it's the latter I'm posting about today, more for the record than anything else in particular. Hey, I was asked, so here goes.

Relative energy demand for steady state road cycling


In the introduction to that item was the chart below, which shows the relative energy demand for steady state cycling for each of the main resistance forces, for gradients ranging from flat (0%) to steep (10%). The idea being to show how the relative importantance of each resistance force varies with zero or positive gradients. The assumptions used are listed in the table on the chart. Click on the chart to see a larger version.


I didn't include negative gradients (descending), as the charting starts gets a little funky - since gravity in that scenario is aiding our forward motion, rather than resisting it. Suffice to say that air resistance is still dominant when going downhill. Besides we tend not to put out nearly as much power going downhills, and brakes also come into play depending on the terrain.

So back to non-negative gradient chit chat.

We can see how air resistance is the dominant resistance force on flatter terrain and hence why aerodynamics matters a lot on flatter and shallower gradients, gradually giving way to gravity which dominates as the climb becomes steeper and our speed slows. This is why weight is an important performance factor on the slopes but much less so on flat ground.

The assumptions used to generate the chart were for a road cyclist in a fairly aerodynamic road bike position, although one would expect them to ride in a less aerodynamic position as the slope steepens. These minor assumption variations don't really change the overall chart trend much.

How about for MTB?


If we change the input assumptions (e.g. CdA, rider mass) all that will happen is the overall trends will shift a bit to the left or right, so having those assumptions precisely right isn't really the point of the chart, it's more about how the components of proportional energy demand vary by gradient.

As an example of modifying those assumptions, a while back for Mountain Bike Magazine, to assist with an article discussing the relative merits of 26" and 19" wheels (see those wheel arguments happen everywhere!), I created the same chart shown below but used alternative input assumptions more akin to MTB riding.

We can see that the same pattern appears as for the road cyclist, however the relative importance of the various forces is somewhat different, and in particular note the larger relative energy demand of rolling resistance due to MTB tyres and rougher terrain. Still, aero still matters in MTB on the shallower terrain, and is something to consider with bike set up depending on the type of racing you do and course profile.


Of course cycle racing is not just about the physics of the energy demand factors or the physiology of energy supply (power output). Skill, experience, strategy, tactics, technical factors and psychology all play their part. This was simply to demonstrate the relative importance of the key physical factors involved.

Get more aero, get lighter, get quality tyres and get fitter. Simple really.

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Saturday, August 24, 2013

Looking under the hood

Today I'm going to take a look under the hood of Functional Threshold Power and explore the relationship between four key underpinning physiological parameters that determine FTP:

  • VO2max
  • Energy yield from aerobic metabolism
  • Efficiency
  • Fractional utilisation of VO2max at threshold
I've prepared a chart (sample below), which I will come back to later to explore this relationship a little more. Those with an existing understanding of the relationship will likely need not look further than the various charts posted, and as normal you can click on them to reveal a larger version.

Scroll down and you will see several versions, showing the relationship between FTP, VO2max and GE at fractional utilisation of 75%, 80%, 85% and 90% of VO2max.

Our maximal sustainable aerobic power is primarily a function of our VO2max,
our gross efficiency, and our fractional utilsation of VO2max at threshold.

For everyone else, I'll introduce and explain the significance of each of these factors and then give an example of how changes affect the power we can sustain.

VO2Max
VO2max testing by the AIS
as reported by Katya Crema


VO2max is a measure of the maximal rate at which we can utilise oxygen. Normally it's also defined by how it is measured, e.g. during an incremental exercise test where the power demand is increased at a specified rate, and how long VO2max is sustained for, so that we don't rely on instantaneous peak values. Measurement of oxygen utilisation requires laboratory testing equipment that records the flow and composition of the body's respiratory gases while performing exercise.

VO2max will typically occur eventually when attempting to sustain a power output above functional threshold, and once reached is typically not sustainable for more than a handful of minutes. How quickly we attain a state of VO2max, and how long we can sustain it are determined by how far above functional threshold power we are attempting to ride, our fitness, power profile and some other individual characteristics.

VO2max is expressed in units of oxygen consumption per unit time, either absolute, i.e. litres of oxygen per minute, or relative to body mass, i.e. millilitres of oxygen per kilogram per minute.

e.g. if a 70kg rider has a VO2max of 60ml/kg/min, it means that for every kilogram of body mass, they can maximally utilise 60 millilitres of oxygen per minute, or 70kg  x  60ml/kg/min  /  1000 ml/litre = 4.2 litres of oxygen per minute.

VO2max sets the ceiling on our aerobic performance capability and as such is a reasonable determinant of our endurance performance potential, however it's not a particularly good predictor of performance. All one can really say is that to be an elite and/or professional cyclist, you will need a relatively high VO2max, typically in excess of 70ml/kg/min, however higher doesn't necessarily mean you will perform better. It just gets you a ticket to the game, but won't necessarily mean you'll be good enough to play.

That's because power output matters far more than how much oxygen we happen to use to generate it, and VO2max is not the sole factor in how much power we are capable of sustaining. And of course there are other factors beyond physiological that determine performance, but all things considered, in endurance cycling power output is a major factor.

VO2max is trainable, although it is also significantly genetically determined (perhaps half), so in a sense, you need to have chosen your parents wisely. You may not see much improvement in absolute VO2max from training, or quite a lot, or something in between. Trainability, which differs by individual and also has a sizeable genetic component, and starting fitness level are big factors. Improvements in VO2max of around 10-25%, can occur in a matter of months. Of course one can attain improvement in VO2max when expressed per unit of body mass simply through weight loss.

There have been some phenomenally high VO2max values occasionally recorded, well into the 90+ ml/kg/min range, with young Norwegian cyclist Oskar Svendsen reported to have the highest recorded VO2 max of 97.5ml/kg/min. Greg Lemond, the American professional cyclist of the 1980s and early 1990s and winner of three Tours de France, was reported to have had a VO2max of 92.5ml/kg/min, and he responded in an interview once that it was in the 92-94ml/kg/min  range. I don't vouch for the validity of these numbers, merely pointing out some of what's been reported.

Energy yield from aerobic metabolism
Citric acid cycle as per wikipedia


Without oxygen we'll die (well duh), and it's critical for sustaining our body's energy production needs, and just like many means of releasing energy through chemical reactions (e.g. rockets, campfires, internal combustion engines and many other chemical reactions), our bodies also use oxygen to help release useful energy from fuel.

Of the biochemical reactions that release energy aerobically (i.e. with oxygen), we utilise two primary fuel sources, one being glycogen and the other free fatty acids (from our body fat stores). Most of the time we obtain energy from both, but when exercising at near threshold power and above, we are heavily, if not solely, reliant on glycogen to meet the energy demand.

Using glycogen as fuel, our body can release around 21.1 kilojoules (kJ) of energy per litre of oxygen. We get a little less from aerobic fat metabolism, around 19.8kJ per litre. So in general the energy released per litre of oxygen utilised is somewhere around 20-21 kJ depending on the mix of fuel substrate used.

We do have the means to also produce energy without oxygen (i.e. anaerobically), however such energy pathways are available to us only for brief periods and are not sustainable, but are good for rapid energy demand (e.g. sprinting) and to supplement the energy provided via aerobic means when the energy demand exceeds our ability to supply via aerobic metabolism alone. Due to the limited supply of such energy though, such efforts are of short duration (seconds to minutes).

Here's a summary of the main energy pathways used by our bodies. It's a fairly complex topic (e.g. just look up the Kreb's Cycle for starters), and is one for the physiologists to chat about over a beer, beer being another key fuel substrate and one of the major food groups, along with burritos, donuts, caffeine and chocolate.

Efficiency


The basic definition of gross efficiency (GE) is the ratio of work done during the specific activity to the total energy expended and expressed as a percentage.

In the case of cycling, GE is the ratio of the energy delivered to the cranks of the bicycle to the total energy metabolised by our body. Sometimes this is referred to as gross mechanical efficiency (GME), just to emphasise the relationship between the mechanical work done at the cranks, to the total energy metabolised by the body.

There are a number of definitions of efficiency in exercise physiology and if you'd like to read about them in a little more depth, then this paper: The reliability of cycling efficiency by Lukh Moseley and Asker Jeukendrup (MSSE 0195-9131/01/3304-0621/$3.00/0 ) is a reasonable place to start and I'm sure there are others. That's just the PubMed extract which doesn't say much about the definitions, but you can find full text version online if you search, and it's a little more instructive.

As highlighted in that paper, trained cyclists typically perform with a GME of around 19-24%, meaning that of the energy metabolised, only about one-fifth to one-quarter actually ends up propelling us forward on a bike. The balance is mostly given off as waste heat, with a little energy of course needed for life support functions!

Have a quick think about that: for every watt you generate at the cranks, you are geneating around another 4 watts of heat. A rider performing longer intervals at 300W is generating somewhere in the vicinity of 1200W of heat! This is precisely why cooling is so vital for performance, as we need to dissipate that excess heat in order to continue to perform at that level.

To measure efficiency we need to measure both our energy output to the bicycle (via a power meter) and our total energy metabolised, which is done via the same respiratory gas exchange analysis equipment used to test VO2max, indeed the two factors are usually measured from the same test. Perhaps one day there will be practical and portable means to measure energy metabolised in the field but for now, the only reliable means is in the lab.

Gross mechanical efficiency can be acutely affected by things such as fatigue, hydration status, glycogen levels, environmental conditions and so on. Chronically we have an efficiency level granted to us by genetic inheritance plus however much we can manage to improve over the course of our cycling careers.

Efficiency is trainable, in particular over the long term, perhaps not to the degree of VO2max or lactate threshold, however other than by performing large volumes of training over many years, it's not totally clear whether or what specific training one can perform to achieve short term improvements. There's lots of noise from many purveyors of a fast performance gains to do with changing pedalling "techniques" or equipment choices, and while some are based on sound science and worth paying attention to, some are far more speculative, while others fall into the snake oil category.

One thought on efficiency is it's related to mix of muscle fibre types, as slower twitch fibres tend to operate with greater efficiency than their faster twitch cousins (which are better at utilising rapid energy release but less efficient metabolism), and so a fast twitch dominant sprinter is more likely to have a lower overall gross effiiency than their diesel mate. The science demonstrating the scope for chronic improvement in efficiency is a bit more limited than for VO2max and lactate threshold, and longitudinal studies are not common.

One thing efficiency is not: it isn't how you pedal, nor the way in which you apply forces to the cranks. It would really help if manufacturers of various cycle training aids would stop misusing the term - it confuses people no end.

Fractional utilisation of VO2max at threshold


This is the percentage of your VO2max you sustain when riding at your functional threshold power. It might range for example from 75% of VO2max to ~ 90% for very fit riders and is an aspect of our fitness and performance that is very trainable over the short to medium term, and can be the element of fitness we gain the greatest improvement from, but it is also something that can be developed over many years of training.

To briefly illustrate, if two riders have an FTP of say 300W, and one is doing so at 80% of their VO2max, while the other at 90% of their VO2max, the rider at 80% of VO2max has quite likely far more scope to further improve their threshold power output.

Ok, so how do all of these factors relate?


There have been suggestions VO2max and efficiency are inversely correlated, although I'm not sure how firm that relationship is, if indeed it exists across the board, or if there is a sound physiological reason why that might be the case.

The maths of the relationship is pretty straighforward though:

FTP = Energy per litre O2 (J)  x  VO2max (ml/kg/min)  x  Fractional VO2max at threshold (%)  x  GME (%)  /  60 (seconds/minute)  / 1000 (ml/litre)

e.g. 
Energy per litre of O2: 20,900 joules (say, refer above for details)
VO2max: 65ml/kg/min (say)
Fractional utilsation of VO2max at threshold: 80% (say)
Gross mechanical efficiency: 22% (say)

FTP = 20,900J  x  65ml/kg/min  x  0.80  x  0.22  /  60,000  =  3.98W/kg

So, now we can see that FTP is a function of those four variables, although we can reasonably assume the energy released per litre of oxygen at threshold is fixed, leaving us three variables to tinker with, and of course you can flip that equation around to ascertain any of the variables chosen given the other factors are known or assumed.

So back to the chart I posted earlier, see this example:
FTP W/kg for a fractional VO2max of 80%.
This rider has scope to improve threshold power by
increasing the fraction of VO2max they can sustain at FTP

On the vertical axis is gross efficiency, the horizontal axis VO2max, and plotted are curves representing various threshold power to body mass ratios, in steps of 0.5W/kg from 2.5W/kg through to 7.0W/kg, for a rider whose threshold power (FTP) occurs at 80% of their VO2max.

So for instance, if a rider has a VO2max of 65ml/kg/min and a GE of 22%, then we can see these intersect at around the 4.0W/kg curve. They could also have the same FTP with higher VO2max but lower efficiency (and vice versa).

If this rider managed to improve their fractional utilisation of VO2max at threshold from 80% up to 90%, then this is what happens with the very same GE and VO2max:
FTP W/kg for a fractional VO2max of 90%.
At same GE and VO2max, rider can sustain a far high power output

All of the W/kg curves have moved down and to the left. Now we can see that the same combination of GE and VO2max results in an FTP of ~4.5W/kg. 
Check the maths: 20,900 x 65 x 0.90 x 0.22 / 60,000 = 4.48W/kg.

To attain the same level of power improvement without increasing fractional VO2max utilisation, it would require an increase in GE from 22% to a little under 25% (not very likely in the short term), or alternatively an increase in their VO2max from 65ml/kg/min to 73.4ml/kg/min.

Of course, one can attain a power improvement via a combination of all three factors, although it's more likely that one will improve VO2max and/or their fractional utlisation of VO2max at threshold in the short to medium term, than attain any short to medium term improvments in gross efficiency.

So what's possible for the freaks exceptionally talented?


The magical troika of high VO2max, high fractional VO2max utilisation at threshold, and a high GE may well be exceptionally rare in the same individual, if it's possible at all, but given a GE of 25% is not exactly unheard of (higher GE values have been reported although some question the validity of those measurements) and we have seen VO2max values reported well beyond 90ml/kg/min, and very fit cyclists will have a fractional VO2max utilisation of ~90% at FTP (I'm not sure if or how much higher that may potentially go), then if we have another look at the above chart and see where a combination of 25% GE and a VO2max of 97.5ml/kg/min intersects, it's beyond the 7.0W/kg curve. It's actually at 7.6W/kg. Yikes.

Of course no-one we know has been measured to have an FTP near that level, certainly no-one without blood/oxygen-vector doping assistance, but just what is actually physiologically possible or plausible? Who really knows?

And what's possible for us mere mortals?


Well have a look at the chart, find a threshold power line near where you are, or would like to be, or perhaps a VO2max value if you happen to know it, and see what various combinations of W/kg, VO2max and GE are required. See what happens at different fractional VO2max utilisation levels.

I dislike setting limits on what's possible, but it's clear that if your highest VO2max is say 60ml/kg/min and there's limited scope for pushing that up much further, then I hate to be the bearer of bad news but you will never see an FTP of 6.0W/kg, but 4.0-4.5W/kg is definitely within reach. 

What does it mean for training?


Well while it's fun to occasionally look under the hood to see what elements of physiology we need to work on to improve our threshold power, one doesn't really need to get too hung up on these individual factors, as they are all inter-related and power measurement not only conveniently condenses the outcome for us but is the primary physiological measure of performance that matters. So keep training hard and smart. There's still no short cuts to improved fitness. There's also no real need to rush out and get your VO2max tested, the power meter will tell you most of what's important.

When you are the limit of  your current improvement in power, then perhaps it might be time to consolidate those gains, and then consider whether a change of tack is necessary to make the next step up. Do you need to give your VO2max a bit more attention, or have you still room to move in lifting your fractional VO2max utilisation? How well do you personally respond to such training?

We can gain some insight into these relationships though inspecting our power profile, and relationships between shorter and longer range power outputs, or for example, how our Functional Threshold Power and our Maximal Aerobic Power relate.

Of course a focus on training to improve one element can and does impact the others, but not always. Perhaps some additional weight loss is required. What one chooses to focus on may be different for a seasoned pro than a local club amateur, but the principles are the same.

OK, enough of that for now - it's time to close the hood, get back into the saddle, and rev the engine!

OK, here are the same charts for each of the fractional VO2max levels I mentiioned earlier:

75% of VO2max:


80% of VO2max:


85% of VO2max:


90% of VO2max:


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